Present techniques to classify and identify radar pulses received by passive Electronic Warfare (EW) detection systems rely primarily on the monopulse measured parameters of carrier frequency, angle of arrival and pulse width as well as the intrapulse measured parameters of items such as pulse repetition interval and scan period. However, modern computer controlled multi-mode radars dynamically vary many of those parameters such as carrier frequency, pulse repetition interval and scan period in any arbitrary manner. Those parameters, as a result, are becoming insufficient to unambiguously discriminate between pulses from multi-mode radars having similar characteristics. Therefore, satisfactory results cannot always be obtained with present approaches to evaluate and classify pulses received from multi-mode radars.
Since it is becoming increasingly difficult to obtain satisfactory results with standard techniques, considerable effort is being directed at the problem of exploiting intrapulse information concerning the nature of the modulation information within radar pulses. Unfortunately, existing approaches to exploit information regarding amplitude and frequency/phase modulation of radar have various limitations since they are often dependent on a particular model of the detected signal. A polynomial model of the signal phase with time, for example, is very good for a linear chirp frequency modulation (quadrature phase) but poorly suited for signals having random discrete frequency modulation.
The need for a signal model can be avoided by directly comparing signals detected by a receiver. Each signal pulse can be compared with previously observed reference signals. When a match is found with one of the reference signals, this will infer that both of those signals were transmitted by the same radar. Otherwise, when a poor match is found between any two signals, it is concluded that a detected signal is transmitted by a new radar.
A simple implementation of this concept for directly comparing signals is to perform frequency demodulation on each signal being compared and, after subtracting the mean of each signal from itself, applying a suitable measurement criteria to determine the amount similarity between the signals. The amount of similarity between the signals will provide an indication of the goodness of the match between signals. The peak of the cross-correlation function has been used for this purpose. This approach has the advantage that carrier frequency offsets between the signals simply result in a shift of the demodulated signals that can easily be removed by subtracting the mean.
The frequency demodulation can be performed by wideband analog frequency demodulators which is a highly developed technology. However, frequency demodulation involves a differentiation of the signal phase and this generally emphasizes noise. A further problem is that signals having frequency modulation which is similar but differs by a scale factor may not be easily distinguishable using cross-correlation. These problems adversely affect the use of frequency demodulators in comparing radar pulses. Since many radars use linear frequency modulation, for instance, it is important to be able to distinguish small differences in the chirp rate.